1. Field of the Invention
The present invention relates to a ZnO-containing semiconductor layer, its manufacture method, and a semiconductor light emitting device, and more particularly to a ZnO-containing semiconductor layer suitable for emission of visual light, its manufacture method, and a semiconductor light emitting device capable of being manufactured by using such a ZnO-containing semiconductor layer as an emission layer.
2. Description of the Related Art
A transition energy between bands of zinc oxide (ZnO) corresponds to an emission energy in a ultraviolet range of about 370 nm. This wavelength is shorter than a visual light range of 400 nm or longer providing a high industrial usage value. Studies have been made to obtain emission at a wavelength longer than 400 nm.
Techniques of narrowing a band gap (gap narrowing) are known as one of the techniques of obtaining emission at a longer wavelength from a ZnO-containing compound semiconductor layer. For example, ZnCdO mixed crystal replacing a portion of Zn with Cd has been proposed in which the band gap can be adjusted in a range of 3.4 eV to 1.5 eV depending on a composition of Cd. However, since Cd is a highly toxic element, adopting ZnCdO mixed crystal is difficult from the viewpoint of safety.
For example, Japanese Patent Laid-open Publication No. 2002-16285 discloses techniques of gap narrowing of a band gap of ZnO by replacing a portion of O with a group VI element of sulfur (S) or selenium (Se) and forming mixed crystal. The techniques are excellent from the viewpoint of safety, as compared to the techniques of introducing Cd.
Japanese Patent Laid-open Publication No. 2002-16285 also discloses a light emitting device by sandwiching a gap-narrowed ZnO-containing compound semiconductor layer (ZnOS layer, ZnOSe layer) between ZnMgO clad layers. According to K. Iwata et al: Phys. Stat. sol (b) 229, No. 2 (2002)887, a relation between an Se composition y and a band gap of ZnO1−ySey (0≦y≦1) is represented by:EZnOse=yEZnSe+(1−y)EZnO−b(1−y)y where EZnOse, EZnSe and EZnO are band gaps of ZnO1−ySey, ZnSe and ZnO, respectively, and b is a bowing parameter. The bowing parameter b is 12.7 eV.
Various techniques have been desired for obtaining emission of visual light from ZnO-containing compound semiconductor. Manufacturing techniques are also desired for manufacturing a semiconductor light emitting device capable of obtaining desired color by using, for example, ZnO-containing compound semiconductor for an emission layer.